1. Field of the Invention
This invention relates to a control device for a hydrostatic gear driven by a drive engine, preferably an internal combustion engine, with an auxiliary pump which also is driven, synchronously, with the drive engine to produce a control pressure which is adjustable via a control valve. The control pressure, which is dependent on the speed of the drive engine, is provided in a control line, which is connected to the setting mechanism of the hydro-pump and/or the hydro-motor of the hydrostatic gear. A pressure reducing valve (i.e. an "inch" valve) is in the control line to eliminate or shut-off of the control pressure depending on the presence of an external parameter. The invention also relates to a setting mechanism to reduce the control pressure when the drive engine attains a predetermined speed, depending on the position of a speed control element of the drive engine.
2. Description of the Prior Art
Control devices with a control valve as above described serve to use the pressure means which, for expediency, is also the working fluid of the hydrostatic gear, for the shifting of the two elements of the hydrostatic gear, i.e. the hydro-pump and/or the hydro-motor. In such case the pressure means is supplied by an auxiliary pump which is driven corresponding to the speed of the drive motor of the hydrostatic gear. Thus, an acceleration of the output shaft of the hydrostatic gear corresponding to the input speed of the gear (i.e. the speed of the drive engine) is possible through a certain control of the supply volume adjustment of the hydro-pump and/or the hydro-motor of the hydrostatic gear. Such a control device has become known, for instance, from the DE-AS 22 47 437, where the control valve is configured as a branching valve of a certain construction described therein where this branching valve, apart from the adjustment of the control pressure dependent on speed, is at the same time configured as a feed valve for the supply of operating fluid for the operating cycle of the hydrostatic gear in order to compensate for leakage losses. Claim 2 of DE-AS 22 47 437 describes that the initial bias force of the spring of the control valve (i.e. the branching valve) can be achieved based on parameters such as the pressure in the operating cycle of the hydrostatic gear, the rated speed of the drive engine, or by similar means. Accordingly it is possible by means of the the control valve to limit the control pressure depending on obtaining the rated speed or depending on the pressure in the operating circuit (i.e. depending on the torque trasmitted by the hydrostatic gear), in order to avoid overloading of the drive engine.
Thus, in the known control device, an operating cylinder of the setting mechanism of the hydro-pump is loaded, by means of the control valve, with a control pressure corresponding to the speed of the drive engine; and thus the hydro-pump of the hydrostatic gear is continously adjusted for a larger supply volume. Accordingly, increasing the rotational speed of the drive engine increases the supply capacity of the hydro-pump and thus increases rotational speed at the output shaft of the hydro-motor of the hydrostatic gear. Loading at the output shaft corresponds to an increase of the operating pressure in the operating cycle. This increasing operating pressure affects the setting mechanism via the hydro-pump and causes a readjustment of the hydro-pump to a smaller supply volume. Thus, a reduction of the supply volume in the hydrostatic gear is determined by two factors:
(1) increasing operating pressure in hydrostatic gear circuit; and PA1 (2) reduction of the control pressure at the setting mechanism of the hydro-pump.
By use of these two factors it is possible to control the hydrostatic gear to maintain a constant transmitted torque.
Those skilled in the art understand that by lowering or reducing the drive engine speed as little as possible one obtains optimum utilization of the engine's drive power. Taking up additional power from the drive engine, for instance by using the hydrostatic gear as a drive means and using additional further operating hydraulic gear also as drive means to which additional further operating hydraulics are connected, can lead to a reduction of the drive motor speed and thus to a reduction of the control pressure as well as a reduction of the supply volume of the hydro-pump. To counteract such speed reduction, it is known to reduce the control pressure by mechanical activation of the control valve independently of the speed of the drive engine. This is known as "inch function." Strictly speaking, this process is called "partial inch function". In such case, for instance, the control valve is mechanically coupled with the accelerator pedal of the drive engine; and, starting with a certain speed along the path of movement of the accelerator pedal, the control curve relating the control pressure to speed is shifted to a lower nominal operating speed. This shift means that an unintentionally large speed reduction through overloading cannot occur. If further power utilization devices are operated by the drive engine, the power reserve thus created is available to the other power utilization devices.
Apart from the "partial inch function" described, those skilled in the art are also aware of "complete inch function." This is achieved either by mechanical activation at the control valve or by an additional pressure reduction valve (i.e. an "inch" valve). Here, the control pressure is shut off either by the control valve or by the "inch" valve, if this is desirable, dependent on an external parameter. This external parameter can be, for instance, a signal, which is given when a brake is activated which slows down the power consuming device driven by the hydrostatic gear. Thus, it is known, for instance, to activate a pressure reduction valve ("inch" valve) with the brake fluid from the braking system of the vehicle, if the drive engine with a hydrostatic gear is used as a drive mechanism in a vehicle.
In summary it will be understood that in control devices of the type described above, "partial inch function" means to set the control pressure at a predetermined level which is lower than that which would correspond to the speed of the drive engine, while "complete inch function" means a complete reduction (i.e. shut-off) of the control pressure, so that the hydro-pump is shifted toward zero supply volume.
In practical applications, the mechanical activation at the control valve dependent on a speed control element of the drive engine to achieve a partial inch function has proven to have many problems. Mechanical activation via rods during usage under severe operating conditions cannot be avoided. Such retro-adjustments outside the manufacturing plant are difficult and unsatisfactory for the complete control device of such an apparatus.
A mechanical activation of the control valve or of a separate pressure reduction valve (i.e. an "inch" valve) to achieve complete reduction ("complete inch function") does not pose any basic problems, since complete reduction ("complete inch function") constitutes a shifting process to shut-off the control pressure and not a control process to shift a control characteristic as is the case with partial reduction (partial "inch function").